A Cosmic First: Ultra-High Energy Neutrinos Found, From Blazing Galaxies Across The Universe

In this artistic rendering, a blazar is accelerating protons that produce pions, which produce neutrinos and gamma rays. Neutrinos are always the result of a hadronic reaction such as the one displayed here. Gamma rays can be produced in both hadronic and electromagnetic interactions.

IceCube/NASA

One of the great mysteries in science is determining not only what's out there, but what creates the signals we detect here on Earth. For over a century, we've known that zipping through the Universe are cosmic rays: high energy particles originating from far beyond our galaxy. While some sources for these particles have been identified, the overwhelming majority of them, including the ones that are most energetic, remain a mystery.

As of today, all of that has changed. The IceCube collaboration, on September 22, 2017, detected an ultra-high-energy neutrino that arrived at the South Pole, and was able to identify its source. When a series of gamma-ray telescopes looked at that same position, they not only saw a signal, they identified a blazar, which happened to be flaring at that very moment. At last, humanity has discovered at least one source that creates these ultra-energetic cosmic particles.

When black holes feed on matter, they create an accretion disk and a bipolar jet perpendicular to it. When a jet from a supermassive black hole points at us, we call it either a BL Lacertae object or a blazar. This is now thought to be a major source of both cosmic rays and high-energy neutrinos.

NASA/JPL

The Universe, everywhere we look, is full of things to look at and interact with. Matter clumps together into galaxies, stars, planets, and even people. Radiation streams through the Universe, covering the entirety of the electromagnetic spectrum. And in every cubic centimeter of space, hundreds of ghostly, tiny-massed particles known as neutrinos can be found.

At least, they could be found, if they interacted with any appreciable frequency with the normal matter we know how to manipulate. Instead, a neutrino would have to pass through a light year of lead to have a 50/50 shot of colliding with a particle in there. For decades after its proposal in 1930, we were unable to detect the neutrino.

Reactor nuclear experimental RA-6 (Republica Argentina 6), en marcha, showing the characteristic Cherenkov radiation from the faster-than-light-in-water particles emitted. The neutrinos (or more accurately, antineutrinos) first hypothesized by Pauli in 1930 were detected from a similar nuclear reactor in 1956.

Centro Atomico Bariloche, via Pieck Darío

In 1956, we first detected them by setting up detectors right outside of nuclear reactors, mere feet away from where neutrinos are produced. In the 1960s, we built large enough detectors — underground, shielded from other contaminating particles — to find the neutrinos produced by the Sun and by cosmic ray collisions with the atmosphere.

Then, in 1987, it was only serendipity that gave us a supernova so close to home that we could detect neutrinos from it. Experiments running for entirely unrelated purposes detected the neutrinos from SN 1987A, ushering in the era of multi-messenger astronomy. Neutrinos, as far as we could tell, traveled across the Universe at energies indistinguishable from the speed of light.

The remnant of supernova 1987a, located in the Large Magellanic Cloud some 165,000 light years away. The fact that neutrinos arrived hours before the first light signal taught us more about the duration it takes light to propagate through the star's layers of a supernova than it did about the speed neutrinos travel at, which was indistinguishable from the speed of light. Neutrinos, light, and gravity appear to all travel at the same speed now.

Noel Carboni & the ESA/ESO/NASA Photoshop FITS Liberator

For some 30 years, the neutrinos from that supernova were the only neutrinos that we had ever confirmed to be from outside of our own Solar System, much less our home galaxy. But that doesn't mean we weren't receiving more distant neutrinos; it simply meant that we couldn't robustly identify them with any known source in the sky. Although neutrinos interact only very weakly with matter, they're more likely to interact if they're higher in energy.

The IceCube observatory, the first neutrino observatory of its kind, is designed to observe these elusive, high-energy particles from beneath the Antarctic ice.

Emanuel Jacobi, IceCube/NSF

Deep within the South Pole ice, IceCube encloses a cubic kilometer of solid material, searching for these nearly-massless neutrinos. When neutrinos pass through the Earth, there's a chance of having an interaction with a particle in there. An interaction will lead to a shower of particles, which should leave unmistakable signatures in the detector.

In this illustration, a neutrino has interacted with a molecule of ice, producing a secondary particle—a muon—that moves at relativistic speed in the ice, leaving a trace of blue light behind it.

Nicolle R. Fuller/NSF/IceCube

In the six years that IceCube has been running, they've detected more than 80 high-energy cosmic neutrinos with energies over 100 TeV: more than ten times the highest energies achieved by any particles at the LHC. Some of them have even crested the PeV scale, achieving energies thousands of times greater than what's needed to create even the heaviest of the known fundamental particles.

Yet despite all these neutrinos of cosmic origin that have arrived on Earth, we haven't yet ever matched them up with a source on the sky that offers a definitive location. Detecting these neutrinos is a tremendous feat, but unless we can correlate them with an actual, observed object in the Universe — for example, that's also observable in some form of electromagnetic light — we have no clue as to what creates them.

When a neutrino interacts in the clear Antarctic ice, it produces secondary particles that leave a trace of blue light as they travel through the IceCube detector.

Nicolle R. Fuller/NSF/IceCube

Theorists have had no problem coming up with ideas, including:

hypernovae, the most superluminous of all the supernovae,

gamma ray bursts,

flaring black holes,

or quasars, the largest, active black holes in the Universe.

But it would take evidence to decide.

An example of a high-energy neutrino event detected by IceCube: a 4.45 PeV neutrino striking the detector back in 2014.

IceCube has been tracking and issuing releases with every ultra-high-energy neutrino they've found. On September 22 of 2017, another such event was seen: IceCube-170922A. In the release that went out, they stated the following:

On 22 Sep, 2017 IceCube detected a track-like, very-high-energy event with a high probability of being of astrophysical origin. The event was identified by the Extremely High Energy (EHE) track event selection. The IceCube detector was in a normal operating state. EHE events typically have a neutrino interaction vertex that is outside the detector, produce a muon that traverses the detector volume, and have a high light level (a proxy for energy).

Cosmic rays shower particles by striking protons and atoms in the atmosphere, but they also emit light due to Cherenkov radiation. By observing both cosmic rays from the sky and neutrinos that strike the Earth, we can use coincidences to uncover the origins of both.

Simon Swordy (U. Chicago), NASA

This endeavor is interesting not just for neutrinos, but for cosmic rays in general. Despite the fact that we've seen millions of cosmic rays of high energies for more than a century, we do not understand where most of them originate. This is true for protons, nuclei, and neutrinos created both at the source and via cascades/showers in the atmosphere.

And that led observers, attempting to perform follow-up observations across the electromagnetic spectrum, to this object.

Artist's impression of the active galactic nucleus. The supermassive black hole at the center of the accretion disk sends a narrow high-energy jet of matter into space, perpendicular to the disc. A blazar about 4 billion light years away is the origin of these cosmic rays and neutrinos.

DESY, Science Communication Lab

This is a blazar: a supermassive black hole that's currently in the active state, feeding on matter and accelerating it to tremendous speeds. Blazars are just like quasars, but with one important difference. While quasars can be oriented in any direction, a blazar will always have one of its jets pointed directly at Earth. They're called blazars because they "blaze" right at you.

About 20 observatories on Earth and in space made follow-up observations of the location where IceCube observed last September's neutrino, which allowed idenification of what scientists deem to be a source of very high energy neutrinos and, thus, of cosmic rays. Besides neutrinos, the observations made across the electromagnetic spectrum included gamma-rays, X-rays, and optical and radio radiation.

Nicolle R. Fuller/NSF/IceCube

Not only that, but when the neutrinos arrived, the blazar was found to be in a flaring state, corresponding to the most active outflows such an object experiences. Since outflows peak and ebb, researchers affiliated with IceCube went through a decade's worth of records prior to the September 22, 2017 flare, and searched for any neutrino events that would originate from the position of TXS 0506+056.

The immediate find? Neutrinos arrived from this object in multiple bursts, spanning many years. By combining neutrino observations with electromagnetic ones, we've robustly been able to establish that high-energy neutrinos are produced by blazars, and that we have the capability to detect them, even from such a great distance. TXS 0506+056, if you were curious, is located some 4 billion light years away.

Blazar TXS 0506+056 is the first identified source of high-energy neutrinos and cosmic rays. This illustration, based on an image of Orion by NASA, shows the location of the blazar, situated in the night sky just off the left shoulder of the constellation Orion. The source is about 4 billion light-years from Earth.

IceCube/NASA/NSF

A tremendous amount can be learned just from this one multi-messenger observation.

Blazars have been demonstrated to be at least one source of cosmic rays.

To produce neutrinos, you need decaying pions, and those are produced by accelerated protons.

This provides the first definitive evidence of proton acceleration by black holes.

This also demonstrates that the blazar TXS 0506+056 is one of the most luminous sources in the Universe.

Finally, from the accompanying gamma rays, we can be certain that cosmic neutrinos and cosmic rays, at least sometimes, have a common origin.

Cosmic rays produced by high-energy astrophysics sources can reach Earth's surface. When a cosmic ray collides with a particle in Earth's atmosphere, it produces a shower of particles that we can detect with arrays on the ground. At last, we've uncovered a major source of them.

ASPERA collaboration / AStroParticle ERAnet

According to Frances Halzen, principal investigator of the IceCube neutrino observatory,

It is interesting that there was a general consensus in the astrophysics community that blazars were unlikely to be sources of cosmic rays, and here we are... The ability to marshal telescopes globally to make a discovery using a variety of wavelengths and coupled with a neutrino detector like IceCube marks a milestone in what scientists call "multi-messenger astronomy."

The era of multi-messenger astronomy is officially here, and now we have three completely independent and complementary ways of looking at the sky: with light, with neutrinos, and with gravitational waves. We've learned that blazars, once considered an unlikely candidate for generating high-energy neutrinos and cosmic rays, in fact create both.

This is an artist's impression of a distant quasar 3C 279. The bipolar jets are a common feature, but it's extremely uncommon for such a jet to be pointed directly at us. When that occurs, we have a Blazar, now confirmed to be a source of both high-energy cosmic rays and the ultra-high-energy neutrinos we've been seeing for years.

ESO/M. Kornmesser

A new scientific field, that of high-energy neutrino astronomy, officially launches with this discovery. Neutrinos are no longer a by-product of other interactions, nor a cosmic curiosity that barely extends beyond our Solar System. Instead, we can use them as a fundamental probe of the Universe and of the basic laws of physics itself. One of the major goals in building IceCube was to identify the sources of high-energy cosmic neutrinos. With the identification of the blazar TXS 0506+056 as the source for both these neutrinos and of gamma rays, that's one cosmic dream that's at last been achieved.

Astrophysicist and author Ethan Siegel is the founder and primary writer of Starts With A Bang! His books, Treknology and Beyond The Galaxy, are available wherever books are sold.